Distribution of M cells in the canine ventricle

Analysis of vulnerability to reentry in acute myocardial ischemia using a realistic human heart model

Electrophysiological alterations of the myocardium caused by acute ischemia constitute a pro-arrhythmic substrate for the generation of potentially lethal arrhythmias. Experimental evidence has shown that the main components of acute ischemia that induce these electrophysiological alterations are hyperkalemia, hypoxia (or anoxia in complete artery occlusion), and acidosis. However, the influence of each ischemic component on the likelihood of reentry is not completely established. Moreover, the role of the His-Purkinje system (HPS) in the initiation and maintenance of arrhythmias is not completely understood. In the present work, we investigate how the three components of ischemia affect the vulnerable window (VW) for reentry using computational simulations. In addition, we analyze the role of the HPS on arrhythmogenesis. A 3D biventricular/torso human model that includes a realistic geometry of the central and border ischemic zones with one of the most electrophysiologically detailed model of ischemia to date, as well as a realistic cardiac conduction system, were used to assess the VW for reentry. Four scenarios of ischemic severity corresponding to different minutes after coronary artery occlusion were simulated. Our results suggest that ischemic severity plays an important role in the generation of reentries. Indeed, this is the first 3D simulation study to show that ventricular arrhythmias could be generated under moderate ischemic conditions, but not in mild and severe ischemia. Moreover, our results show that anoxia is the ischemic component with the most significant effect on the width of the VW. Thus, a change in the level of anoxia from moderate to severe leads to a greater increment in the VW (40 ms), in comparison with the increment of 20 ms and 35 ms produced by the individual change in the level of hyperkalemia and acidosis, respectively. Finally, the HPS was a necessary element for the generation of approximately 17% of reentries obtained. The retrograde conduction from the myocardium to HPS in the ischemic region, conduction blocks in discrete sections of the HPS, and the degree of ischemia affecting Purkinje cells, are suggested as mechanisms that favor the generation of ventricular arrhythmias.

Personalized Cardiac Computational Models: From Clinical Data to Simulation of Infarct-Related Ventricular Tachycardia

In the chronic stage of myocardial infarction, a significant number of patients develop life-threatening ventricular tachycardias (VT) due to the arrhythmogenic nature of the remodeled myocardium. Radiofrequency ablation (RFA) is a common procedure to isolate reentry pathways across the infarct scar that are responsible for VT. Unfortunately, this strategy show relatively low success rates; up to 50% of patients experience recurrent VT after the procedure. In the last decade, intensive research in the field of computational cardiac electrophysiology (EP) has demonstrated the ability of three-dimensional (3D) cardiac computational models to perform in-silico EP studies. However, the personalization and modeling of certain key components remain challenging, particularly in the case of the infarct border zone (BZ). In this study, we used a clinical dataset from a patient with a history of infarct-related VT to build an image-based 3D ventricular model aimed at computational simulation of cardiac EP, including detailed patient-specific cardiac anatomy and infarct scar geometry. We modeled the BZ in eight different ways by combining the presence or absence of electrical remodeling with four different levels of image-based patchy fibrosis (0, 10, 20, and 30%). A 3D torso model was also constructed to compute the ECG. Patient-specific sinus activation patterns were simulated and validated against the patient's ECG. Subsequently, the pacing protocol used to induce reentrant VTs in the EP laboratory was reproduced in-silico. The clinical VT was induced with different versions of the model and from different pacing points, thus identifying the slow conducting channel responsible for such VT. Finally, the real patient's ECG recorded during VT episodes was used to validate our simulation results and to assess different strategies to model the BZ. Our study showed that reduced conduction velocities and heterogeneity in action potential duration in the BZ are the main factors in promoting reentrant activity. Either electrical remodeling or fibrosis in a degree of at least 30% in the BZ were required to initiate VT. Moreover, this proof-of-concept study confirms the feasibility of developing 3D computational models for cardiac EP able to reproduce cardiac activation in sinus rhythm and during VT, using exclusively non-invasive clinical data.

Optimization of Lead Placement in the Right Ventricle During Cardiac Resynchronization Therapy. A Simulation Study

Patients suffering from heart failure and left bundle branch block show electrical ventricular dyssynchrony causing an abnormal blood pumping. Cardiac resynchronization therapy (CRT) is recommended for these patients. Patients with positive therapy response normally present QRS shortening and an increased left ventricle (LV) ejection fraction. However, around one third do not respond favorably. Therefore, optimal location of pacing leads, timing delays between leads and/or choosing related biomarkers is crucial to achieve the best possible degree of ventricular synchrony during CRT application. In this study, computational modeling is used to predict the optimal location and delay of pacing leads to improve CRT response. We use a 3D electrophysiological computational model of the heart and torso to get insight into the changes in the activation patterns obtained when the heart is paced from different regions and for different atrioventricular and interventricular delays. The model represents a heart with left bundle branch block and heart failure, and allows a detailed and accurate analysis of the electrical changes observed simultaneously in the myocardium and in the QRS complex computed in the precordial leads. Computational simulations were performed using a modified version of the O'Hara et al. action potential model, the most recent mathematical model developed for human ventricular electrophysiology. The optimal location for the pacing leads was determined by QRS maximal reduction. Additionally, the influence of Purkinje system on CRT response was assessed and correlation analysis between several parameters of the QRS was made. Simulation results showed that the right ventricle (RV) upper septum near the outflow tract is an alternative location to the RV apical lead. Furthermore, LV endocardial pacing provided better results as compared to epicardial stimulation. Finally, the time to reach the 90% of the QRS area was a good predictor of the instant at which 90% of the ventricular tissue was activated. Thus, the time to reach the 90% of the QRS area is suggested as an additional index to assess CRT effectiveness to improve biventricular synchrony.

Fully coupled fluid-electro-mechanical model of the human heart for supercomputers

In this work, we present a fully coupled fluid-electro-mechanical model of a 50th percentile human heart. The model is implemented on Alya, the BSC multi-physics parallel code, capable of running efficiently in supercomputers. Blood in the cardiac cavities is modeled by the incompressible Navier-Stokes equations and an arbitrary Lagrangian-Eulerian (ALE) scheme. Electrophysiology is modeled with a monodomain scheme and the O'Hara-Rudy cell model. Solid mechanics is modeled with a total Lagrangian formulation for discrete strains using the Holzapfel-Ogden cardiac tissue material model. The three problems are simultaneously and bidirectionally coupled through an electromechanical feedback and a fluid-structure interaction scheme. In this paper, we present the scheme in detail and propose it as a computational cardiac workbench.

The role of late I Na in development of cardiac arrhythmias

Late I Na is an integral part of the sodium current, which persists long after the fast-inactivating component. The magnitude of the late I Na is relatively small in all species and in all types of cardiomyocytes as compared with the amplitude of the fast sodium current, but it contributes significantly to the shape and duration of the action potential. This late component had been shown to increase in several acquired or congenital conditions, including hypoxia, oxidative stress, and heart failure, or due to mutations in SCN5A, which encodes the α-subunit of the sodium channel, as well as in channel-interacting proteins, including multiple β subunits and anchoring proteins. Patients with enhanced late I Na exhibit the type-3 long QT syndrome (LQT3) characterized by high propensity for the life-threatening ventricular arrhythmias, such as Torsade de Pointes (TdP), as well as for atrial fibrillation. There are several distinct mechanisms of arrhythmogenesis due to abnormal late I Na, including abnormal automaticity, early and delayed after depolarization-induced triggered activity, and dramatic increase of ventricular dispersion of repolarization. Many local anesthetic and antiarrhythmic agents have a higher potency to block late I Na as compared with fast I Na. Several novel compounds, including ranolazine, GS-458967, and F15845, appear to be the most selective inhibitors of cardiac late I Na reported to date. Selective inhibition of late I Na is expected to be an effective strategy for correcting these acquired and congenital channelopathies.

A reappraisal on lidocaine-sensitive repetitive, uniform atrial tachycardia

BACKGROUND

Lidocaine sensitive, repetitive atrial tachycardia is an unusual arrhythmia whose electrophysiologic substrate remains undefined. We aimed to analyze the electropharmacologic characteristics of this arrhythmia with emphasis on its cellular substrate and response to drug challenges.

METHODS

We retrospectively analyzed a series of 18 patients from an electrocardiographic and electrophysiologic perspective and the response to pharmacological challenge.

RESULTS

There was no evidence of structural heart disease in 12 patients, 4 patients presented with systemic hypertension; one patient had a prior myocardial infarction and one a mitral valve prolapse. The arrhythmia depicted a consistent pattern in nine patients. The first initiating ectopic beat showed a long coupling interval, the cycle length of the second atrial ectopic beat presented the shortest cycle length and a further prolongation was apparent towards the end of the atrial salvos. Conversely, in the other nine cases, the atrial tachycardia cycle length was erratic. The arrhythmia was suppressed by asynchronous atrial pacing at cycle lengths longer than those of the atrial tachycardia. Intravenous lidocaine eliminated the arrhythmia in all patients, but intravenous verapamil suppressed the atrial tachycardia in only two patients while adenosine caused a transient disappearance in 2/8 patients. Only one patient responded to all the three agents. Radiofrequency ablation was successfully performed in 10 patients.

CONCLUSIONS

Repetitive uniform atrial tachycardia can be sensitive to lidocaine. In few cases, this rare focal arrhythmia may be also suppressed by adenosine and/or verapamil, which suggests a diversity of electrophysiologic substrates that deserve to be accurately identified.

Nifekalant enlarged the transmural activation-recovery interval difference as well as the peak-to-end interval on surface ECG in a patient with short-QT syndrome

A 38-year-old woman with type 1 short-QT syndrome (SQTS) was referred to our hospital. Her ECG showed short QT/QTc interval and peaked T wave. Activation-recovery intervals (ARIs) were calculated from the intracardiac endocardial and epicardial electrode catheters placed in the left ventricle (LV). Intravenous administration of nifekalant prolonged effective refractory period at multiple ventricular sites as well as the QT/QTc interval (from 260/300 to 364/419 ms) on the surface ECG. Nifekalant also enlarged the transmural ARI dispersion of the ventricular repolarization, which was measured by the difference between the longest endocardial ARI and the shortest epicardial ARI during atrial pacing at 90 bpm, from 73 to 103-105 ms. These values corresponded to the intervals between the peak and end of the T wave on the surface ECG. Nifekalant-induced QT interval prolongation on the surface ECG may not indicate attenuation of the arrhythmogenic potential in the heart of SQTS patients.

Distinct pharmacologic substrate in lidocaine-sensitive, repetitive atrial tachycardia

Lidocaine-sensitive, repetitive atrial tachycardia is an uncommon arrhythmia. The electrophysiologic substrate is still unknown, and the pharmacologic responses have not been fully explored. The aim of this study was to investigate the effects of intravenous adenosine and verapamil in patients with lidocaine-sensitive atrial tachycardia. In 9 patients with repetitive uniform atrial tachycardia, the response to intravenous adenosine (12 mg), lidocaine (1 mg/kg body weight), and verapamil (10 mg) were sequentially investigated. Simultaneous 12-lead electrocardiogram (ECG) was recorded at baseline and continuously monitored thereafter. Tracings were obtained at regularly timed intervals right after the administration of each drug to evaluate changes in the arrhythmia characteristics. Repetitive atrial tachycardia was abolished by intravenous lidocaine in the 9 patients within the first 2 minutes after the end of injection. Adenosine suppressed the arrhythmia in 2 patients and shortened the runs of atrial ectopic activity in 1 patient, while verapamil was effective in 2 patients, 1 of them insensitive to adenosine and the other 1 sensitive to this agent. In 5 patients, the arrhythmia was abolished by radiofrequency ablation at different sites of the right atrium. Lidocaine-sensitive atrial tachycardia may eventually be also suppressed by adenosine and/or verapamil. This suggests that this enigmatic arrhythmia may be caused by different underlying electrophysiologic substrates and that at least in some cases, delayed afterdepolarizations seem to play a determining role.

Effect of activation sequence on transmural patterns of repolarization and action potential duration in rabbit ventricular myocardium

Although transmural heterogeneity of action potential duration (APD) is established in single cells isolated from different tissue layers, the extent to which it produces transmural gradients of repolarization in electrotonically coupled ventricular myocardium remains controversial. The purpose of this study was to examine the relative contribution of intrinsic cellular gradients of APD and electrotonic influences to transmural repolarization in rabbit ventricular myocardium. Transmural optical mapping was performed in left ventricular wedge preparations from eight rabbits. Transmural patterns of activation, repolarization, and APD were recorded during endocardial and epicardial stimulation. Experimental results were compared with modeled data during variations in electrotonic coupling. A transmural gradient of APD was evident during endocardial stimulation, which reflected differences previously seen in isolated cells, with the longest APD at the endocardium and the shortest at the epicardium (endo: 165 ± 5 vs. epi: 147 ± 4 ms; P < 0.05). During epicardial stimulation, this gradient reversed (epi: 162 ± 4 vs. endo: 148 ± 6 ms; P < 0.05). In both activation sequences, transmural repolarization followed activation and APD shortened along the activation path such that significant transmural gradients of repolarization did not occur. This correlation between transmural activation time and APD was recapitulated in simulations and varied with changes in intercellular coupling, confirming that it is mediated by electrotonic current flow between cells. These data suggest that electrotonic influences are important in determining the transmural repolarization sequence in rabbit ventricular myocardium and that they are sufficient to overcome intrinsic differences in the electrophysiological properties of the cells across the ventricular wall.

Effects of bepridil versus E-4031 on transmural ventricular repolarization and inducibility of ventricular tachyarrhythmias in the dog

BACKGROUND

Bepridil (a multiple channel blocker) may markedly prolong the QT interval and induce polymorphic ventricular tachyarrhythmias (VTA). We compared the transmural ventricular repolarization characteristics and inducibility of polymorphic VTA after administration of bepridil versus the pure I(Kr) blocker, E-4031, each administered to five open-chest dogs.

METHODS

We used plunge needle electrode to record transmural left ventricular (LV) repolarization and activation-recovery interval (ARI) to estimate local repolarization. The correlation between paced cycle length and ARI was separately examined in the LV endocardium, mid-myocardium (Mid), and epicardium. Attempts to induce VTA were made during bradycardia and sympathetic stimulation.

RESULTS

Bepridil and E-4031 prolonged QT interval and ARI in all LV layers, though the magnitude of prolongation was greatest in Mid, increasing the transmural ARI dispersion, particularly during bradycardia. Compared with E-4031, bepridil caused mild, reverse use-dependent changes in ventricular repolarization, and less ARI dispersion than E-4031 during slow ventricular pacing. Both drugs increased ARI(max) and cycle length at 50% of ARI(max), though the changes were smaller after bepridil than after E-4031 administration. Bradycardia after the administration of each drug induced no VTA; however, sympathetic stimulation induced sustained polymorphic VTA in two of five dogs treated with E-4031 versus no dog treated with bepridil.

CONCLUSIONS

Unlike the pure I(kr) blocker, E-4031, bepridil exhibited weak properties of reverse use-dependency and protected against sympathetic stimulation-induced VTA. It may be an effective supplemental treatment for recipients of implantable cardioverter defibrillator.

Molecular mechanisms of adverse drug reactions in cardiac tissue

The myocardium is the target of toxicity for a number of drugs. Based on pharmacological evidence, cellular targets for drugs that produce adverse reactions can be categorized into a number of sites that include the cell membrane-bound receptors, the second messenger system, ionic channels, ionic pumps, and intracellular organelles. Additionally, interference with the neuronal input to the heart can also present a global site where adverse drug effects can manifest themselves. Simply, a drug can interfere with the normal cardiac action by modifying an ion channel function at the plasma membrane level leading to abnormal repolarization and/or depolarization of the heart cells thus precipitating a disruption in the rhythm and causing dysfunction in contractions and/or relaxations of myocytes. It is now recognized that toxic actions of drugs against the myocardium are not exclusive to the antitumor or the so-called cardiac drugs, and many other drugs with diverse chemical structures, such as antimicrobial, antimalarial, antihistamines, psychiatric, and gastrointestinal medications, seem to be capable of severely compromising myocardium function. At present, great emphasis in terms of drug safety is being placed on the interaction of many classes of drugs with the hERG potassium channel in cardiac tissue. The interest in the latter channel stems from the simplified view that drugs that block the hERG potassium channel cause prolongation of the QT interval, and this can cause life-threatening cardiac arrhythmias. Based on the evidence in the current literature, this concept does not seem to always hold true.

Transseptal dispersion of repolarization and its role in the development of Torsade de Pointes arrhythmias

OBJECTIVE

This study was designed to quantitate transseptal dispersion of repolarization (DR) and delineate its role in arrhythmogenesis using the calcium agonist BayK 8644 to mimic the gain of function of calcium channel current responsible for Timothy syndrome.

BACKGROUND

Amplification of transmural dispersion of repolarization (TDR) has been shown to contribute to development of Torsade de Pointes (TdP) arrhythmias under long-QT conditions.

METHODS

An arterially perfused septal wedge preparation was developed via cannulation of the septal artery. Action potentials (APs) were recorded using floating microelectrodes together with a transseptal electrocardiogram (ECG). These data were compared to those recorded from arterially perfused canine left ventricular (LV) wedge preparations.

RESULTS

Under control conditions, the shortest AP duration measured at 90% repolarization (APD(90)) was observed in right ventricular (RV) endocardium (181.8 +/- 15 ms), APD(90) peaked close to midseptum (278.0 +/- 32 ms), and abbreviated again as LV endocardium was approached (207.3 +/- 9 ms). Transseptal DR averaged 106 +/- 24 ms and T(peak)-T(end) 84 +/- 7 ms (n = 6). TDR and T(peak)-T(end) recorded from LV wedge were 36 +/- 9 ms and 34 +/- 19 ms, respectively (n = 30). BayK 8644 increased transseptal DR to 123.2 +/- 35 ms (n = 5) and induced early and delayed afterdepolarizations (3/5), rate-dependent ST-T-wave alternans (5/5), and TdP arrhythmias (3/5).

CONCLUSIONS

Our data indicate that dispersion of repolarization across the interventricular septum is twice that of the LV free wall, predisposing to development of TdP under long-QT conditions. Our findings suggest that the coronary-perfused ventricular septal preparation may be a sensitive model in which to assess the potential arrhythmogenic effects of drugs and pathophysiological conditions.

Intramural Purkinje cell network of sheep ventricles as the terminal pathway of conduction system

To identify the anatomical basis for cardiac electrical signal conduction, particularly seeking the intramural terminals of conduction pathway within the ventricles, sheep hearts were examined compared with human hearts utilizing the characteristic morphology of Purkinje cells as a histological marker. In 15 sheep and five human autopsies of noncardiac death, prevalence of Purkinje or Purkinje-type cells were histologically examined in the atrioventricular node, its distal conduction pathway, the interventricular septum, and the right- and left-ventricular free walls. Myocardial tissue cleavages were examined in the transmural sections (along cardiac base-to-apex axis) obtained from the septum and ventricular free walls. Serial histological sections through virtually the entirety of the septum in selected sheep were used as the basis of a three-dimensional reconstruction of the conduction pathway, particularly of the intramural Purkinje cell network. Purkinje cells were found within the mural myocardium of sheep ventricles whereas no intramural Purkinje-type cell was detected within the human ventricles. In the sheep septum, every intramural Purkinje cell composed a three-dimensional network throughout the mural myocardium, which proximally connected to the subendocardial extension of the bundle branches and distally formed an occasional junction with ordinary working myocytes. The Purkinje-cell network may participate in the ventricular excitation as the terminal conduction pathway. Individual connections among the Purkinje cells contain the links of through-wall orientation which would benefit the signal conduction crossing the architectural barriers by cleavages in sheep hearts. The myocardial architectural changes found in diseased hearts could disrupt the network links including those with transmural orientation. Anat Rec, 2009. (c) 2008 Wiley-Liss, Inc.

Effect of transmurally heterogeneous myocyte excitation-contraction coupling on canine left ventricular electromechanics

The excitation-contraction coupling properties of cardiac myocytes isolated from different regions of the mammalian left ventricular wall have been shown to vary considerably, with uncertain effects on ventricular function. We embedded a cell-level excitation-contraction coupling model with region-dependent parameters within a simple finite element model of left ventricular geometry to study effects of electromechanical heterogeneity on local myocardial mechanics and global haemodynamics. This model was compared with one in which heterogeneous myocyte parameters were assigned randomly throughout the mesh while preserving the total amount of each cell subtype. The two models displayed nearly identical transmural patterns of fibre and cross-fibre strains at end-systole, but showed clear differences in fibre strains at earlier points during systole. Haemodynamic function, including peak left ventricular pressure, maximal rate of left ventricular pressure development and stroke volume, were essentially identical in the two models. These results suggest that in the intact ventricle heterogeneously distributed myocyte subtypes primarily impact local deformation of the myocardium, and that these effects are greatest during early systole.

Acute ischemia of canine interventricular septum produces asymmetric suppression of conduction

BACKGROUND

Acute ischemia depresses tissue excitability more rapidly in the epicardium than in the endocardium of the canine left ventricular (LV) free wall. However, the effects of acute ischemia on conduction in the interventricular septum (IVS), which is composed of right ventricular (RV) and LV endocardium and midmyocardium without epicardium, are less known.

OBJECTIVE

The purpose of this study was to evaluate the hypothesis that the IVS exhibits transseptal differences in local tissue response to acute ischemia.

METHODS

Isolated canine IVS preparations were perfused through the septal branch of the anterior descending coronary artery, and conduction on the cut-exposed transseptal surfaces was optically mapped before and after two sequential episodes of 8 minutes of global ischemia (separated by >60 minutes of reperfusion). The preparations were paced alternately between the RV endocardium and LV endocardium at cycle lengths of 250, 300, and 1,500 ms.

RESULTS

Prior to ischemia, transseptal conduction was radial and symmetric during either RV endocardial or LV endocardial pacing at all cycle lengths. Eight minutes of ischemia depressed conduction velocity more in the RV half than in the LV half of the IVS and caused local conduction block in the sub-RV endocardium, especially during rapid pacing. The K(ATP) channel blocker glibenclamide (10 micromol/L) prevented development of this transseptal asymmetry and conduction block during ischemia.

CONCLUSION

Acute global ischemia increased transseptal heterogeneity and induced sub-RV endocardial block of conduction via activation of the ATP-sensitive potassium current. Such changes could contribute to initiation of arrhythmia in patients with septal infarction.

Ionic, molecular, and cellular bases of QT-interval prolongation and torsade de pointes

Torsade de pointes (TdP) is a life-threatening arrhythmia that develops as a consequence of a reduction in the repolarization reserve of cardiac cells leading to amplification of electrical heterogeneities in the ventricular myocardium as well as to the development of early after depolarization-induced triggered activity. Electrical heterogeneities within the ventricles are due to differences in the time course of repolarization of the three predominant cell types that make up the ventricular myocardium, giving rise to transmural voltage gradients and a dispersion of repolarization that contributes to the inscription of the electrocardiographic T wave. A number of non-antiarrhythmic drugs and antiarrhythmic agents with class III actions and/or the various mutations and cardiomyopathies associated with the long QT syndrome reduce net repolarizing current and amplify spatial dispersion of repolarization, thus creating the substrate for re-entry. This results in a prolongation of the QT interval, abnormal T waves, and development of TdP. Agents that prolong the QT interval but do not cause an increase in transmural dispersion of repolarization (TDR) do not induce TdP, suggesting that QT prolongation is not the sole or optimal determinant for arrhythmogenesis. This article reviews recent advances in our understanding of these mechanisms, particularly the role of TDR in the genesis of drug-induced TdP, and examines how these may guide us towards development of safer drugs.

Development of a coronary-perfused interventricular septal preparation as a model for studying the role of the septum in arrhythmogenesis

BACKGROUND

Coronary-perfused ventricular wedge preparations have proven valuable in the elucidation of the mechanisms of arrhythmias. This study was undertaken to develop an arterially perfused model of the interventricular (IV) septum.

METHODS

A canine septal preparation was developed via cannulation of the septal artery. Action potentials were recorded from ventricular endocardial surfaces and locations within the septum using floating microelectrodes; a transseptal electrocardiogram was simultaneously recorded. In some experiments, the calcium agonist BayK 8644 was used to enhance transseptal heterogeneity of action potential (AP) duration.

RESULTS

Distinctive electrocardiographic waveforms and dissimilar AP morphologies and durations were observed across the IV septum. The range of AP durations observed exceeded that found in the left ventricular wedge. BayK 8644 further accentuated these differences and induced torsades de pointes arrhythmias.

CONCLUSIONS

The arterially perfused septal preparation is a sensitive model for the study of arrhythmias that may arise from the IV septum.

Optical measurements of intramural action potentials in isolated porcine hearts using optrodes

BACKGROUND

Measurements of intramural membrane potential (Vm) would greatly increase knowledge of cardiac arrhythmias and defibrillation. Optrodes offer the possibility for three-dimensional Vm mapping, but their signal quality has been inadequate.

OBJECTIVE

The purpose of this work was to improve optrode signal quality and use optrodes to measure intramural distribution of action potentials and shock-induced Vm changes in porcine hearts.

METHODS

Optrodes were made from seven optical fibers 225 or 325 microm in diameter. Fiber ends were polished at a 45 degrees angle, which improved light collection and allowed their insertion without a needle. Fluorescent measurements were performed in isolated porcine hearts perfused with Tyrode's solution or blood using Vm-sensitive dye RH-237 and a 200-W Hg/Xe lamp.

RESULTS

The signal-to-noise ratio for 325-microm fibers was 44 +/- 23 in blood-perfused hearts (n = 5) and 106 +/- 45 in Tyrode's-perfused hearts (n = 3), which represents an approximately four-fold improvement over previously reported data. There was close correspondence between optical and electrical measurements of activation times and action potential duration (APD). No significant intramural APD gradients were observed at cycle lengths up to 4 s and in the presence of dofetilide or d-sotalol. Application of shocks (5-50 V/cm) produced large intramural Vm changes (up to approximately 200% action potential amplitude), possibly reflecting a combined effect of tissue discontinuities and optrode geometry.

CONCLUSIONS

A substantial improvement of optrode signal quality was achieved. Optical measurements of APD and activation times matched electrical measurements. Optrode measurements revealed no significant intramural APD gradients. Application of shocks caused large intramural Vm changes that could be influenced by the optrode geometry.

Heterogeneity and cardiac arrhythmias: an overview

This lecture examines the hypothesis that amplification of spatial dispersion of repolarization in the form of transmural dispersion of repolarization (TDR) underlies the development of life-threatening ventricular arrhythmias associated with inherited ion channelopathies, including the long QT, short QT, and Brugada syndromes as well as catecholaminergic polymorphic ventricular tachycardia. In the long QT syndrome, amplification of TDR often is secondary to preferential prolongation of the action potential duration of M cells, whereas in Brugada syndrome, it is thought to be due to selective abbreviation of the action potential duration of right ventricular epicardium. In the short QT syndrome, preferential abbreviation of action potential duration of either endocardium or epicardium appears to be responsible for amplification of TDR. In catecholaminergic polymorphic ventricular tachycardia, reversal of the direction of activation of the ventricular wall is responsible for the increase in TDR. Thus, the long QT, short QT, Brugada, and catecholaminergic ventricular tachycardia syndromes are pathologies with very different phenotypes and etiologies. However, these syndromes share a common final pathway in their predisposition to sudden cardiac death.

Role of spatial dispersion of repolarization in inherited and acquired sudden cardiac death syndromes

This review examines the role of spatial electrical heterogeneity within the ventricular myocardium on the function of the heart in health and disease. The cellular basis for transmural dispersion of repolarization (TDR) is reviewed, and the hypothesis that amplification of spatial dispersion of repolarization underlies the development of life-threatening ventricular arrhythmias associated with inherited ion channelopathies is evaluated. The role of TDR in long QT, short QT, and Brugada syndromes, as well as catecholaminergic polymorphic ventricular tachycardia (VT), is critically examined. In long QT syndrome, amplification of TDR is often secondary to preferential prolongation of the action potential duration (APD) of M cells; in Brugada syndrome, however, it is thought to be due to selective abbreviation of the APD of the right ventricular epicardium. Preferential abbreviation of APD of the endocardium or epicardium appears to be responsible for the amplification of TDR in short QT syndrome. In catecholaminergic polymorphic VT, reversal of the direction of activation of the ventricular wall is responsible for the increase in TDR. In conclusion, long QT, short QT, Brugada, and catecholaminergic polymorphic VT syndromes are pathologies with very different phenotypes and etiologies, but they share a common final pathway in causing sudden cardiac death.

The role of sodium channel current in modulating transmural dispersion of repolarization and arrhythmogenesis

Ventricular myocardium in larger mammals is composed of three distinct cell types: epicardial, M, and endocardial cells. Epicardial and M cell, but not endocardial cell, action potentials have a prominent I(to)-mediated notch. M cells are distinguished from the other cell types in that they display a smaller I(Ks), but a larger late I(Na) and I(Na-Ca). These ionic differences may account for the longer action potential duration (APD) and steeper APD-rate relationship of the M cell. The difference in the time course of repolarization of phase 1 and phase 3 contributes to the inscription of the electrocardiographic J wave and T wave, respectively. These repolarization gradients are modulated by electrotonic interactions, [K(+)](o), and agents or mutations that alter net repolarizing current. An increase in late I(Na), as occurring under a variety of pathophysiological states or in response to certain toxins, leads to a preferential prolongation of the M cell action potential, thus prolonging the QT interval and increasing transmural dispersion of repolarization (TDR), which underlies the development of torsade de pointes (TdP) arrhythmias. Agents that reduce late I(Na) are effective in reducing TDR and suppressing TdP. A reduction in peak I(Na) or an increase in net repolarizing current in the early phases of the action potential can lead to a preferential abbreviation of the action potential of epicardium in the right ventricle, and thus the development of a large TDR, phase 2 reentry, and polymorphic ventricular tachycardia associated with the Brugada syndrome.

Role of Autonomic Nervous Activity in the Antiarrhythmic Effects of Magnesium Sulfate in a Canine Model of Polymorphic Ventricular Tachyarrhythmia Associated with Prolonged QT Interval

This study was performed to examine the role played by the autonomic nervous system in the antiarrhythmic effects of magnesium sulfate (Mg) in a canine model of polymorphic ventricular tachyarrhythmia facilitated by anthopleurin-A and a slower heart rate induced QT interval prolongation. In 6 experiments, complete atrioventricular block was created to control the heart rate and bradycardia at 800- to 1500-ms cycle lengths was applied for 60 sec before and after drug-induced autonomic block. Transmural unipolar electrograms were recorded from multipolar needle electrodes, and activation-recovery intervals (ARI) were measured. Before drug-induced autonomic block, polymorphic ventricular tachyarrhythmia developed in all 6 experiments during bradycardia before but not after the administration of Mg (0.2 ml/kg intravenous bolus). During drug-induced autonomic block, triggered premature activity decreased without significant changes in underlying dispersion of repolarization and polymorphic ventricular tachyarrhythmia developed during bradycardia in 1 experiment. Administration of Mg during drug-induced autonomic block eliminated premature activity and polymorphic ventricular tachyarrhythmia during bradycardia. The distribution of left ventricular (LV) and right ventricular repolarization and dispersion of transmural repolarization were analyzed before and 60 sec after Mg administration during ventricular pacing at 80 bpm. Mg caused a modest shortening of ARI at all sites before and after drug-induced autonomic block. Since ARI shortening was greater at the mid-myocardial sites than at other LV sites, Mg decreased transmural ARI dispersion from 77 +/- 16 to 46 +/- 21 ms before drug-induced autonomic block and from 79 +/- 7 to 51 +/- 16 ms after drug-induced autonomic block. The antiarrhythmic effects of Mg in this model of long QT syndrome were attributable to its direct pharmacological properties and not to changes in ambient autonomic nervous activity. The blockade of sympathetic activity decreased the incidence of premature events and partially suppressed polymorphic ventricular tachyarrhythmia in this model.

Assessing the proarrhythmic potential of drugs: current status of models and surrogate parameters of torsades de pointes arrhythmias

Torsades de pointes (TdP) is a potentially lethal cardiac arrhythmia that can occur as an unwanted adverse effect of various pharmacological therapies. Before a drug is approved for marketing, its effects on cardiac repolarisation are examined clinically and experimentally. This paper expresses the opinion that effects on repolarisation duration cannot directly be translated to risk of proarrhythmia. Current safety assessments of drugs only involve repolarisation assays, however the proarrhythmic profile can only be determined in the predisposed model. The availability of these proarrhythmic animal models is emphasised in the present paper. It is feasible for the pharmaceutical industry to establish one or more of these proarrhythmic animal models and large benefits are potentially available if pharmaceutical industries and patient-care authorities embraced these models. Furthermore, suggested surrogate parameters possessing predictive power of TdP arrhythmia are reviewed. As these parameters are not developed to finalisation, any meaningful study of the proarrhythmic potential of a new drug will include evaluation in an integrated model of TdP arrhythmia.

Effects of amiodarone on transmural dispersion of ventricular effective refractory periods across myocardial layers in the normal and hypertrophic canine heart

The effects of amiodarone on transmural dispersion of ventricular effective refractory periods (ERPs) in the normal and hypertrophic canine heart were investigated in vivo. By using the programmed stimulation protocol, the ERPs of epicardium (Epi), midmyocardium (Mid) and endocardium (Endo) were measured by inserting specially-designed electrodes into the three myocardial layers before and after mainlining of amiodarone. No significant ERPs-dispersion was observed in the three layers before and after mainlining of amiodarone in the normal group. In contrast, ERPs of all the three layers were prolonged in the hypertrophic heart, while the ERPs-dispersion was reduced significantly after mainlining of amiodarone. The ERPs-dispersion was significantly increased in the hypertrophic heart but not in the normal heart using "long-short" and "short-long" interval stimulation technique. It was concluded that (1) the differences in ERPs-dispersion among the three layers were significant in hypertrophic heart, and differences were not significant in normal canine heart; (2) ERPs of each three-myocardial layers were significantly prolonged after using amiodarone, but the ERPs-dispersion decreased in hypertrophic heart and (3) the programmed extrastimulus technique of "long-short" and "short-long" intervals increased the transmural ERPs-dispersion in the hypertrophic heart.

Amplification of spatial dispersion of repolarization underlies sudden cardiac death associated with catecholaminergic polymorphic VT, long QT, short QT and Brugada syndromes

This review examines the hypothesis that amplification of spatial dispersion of repolarization in the form of transmural dispersion of repolarization (TDR) underlies the development of life-threatening ventricular arrhythmias associated with inherited ion channelopathies including the long QT, short QT and Brugada syndromes as well as catecholaminergic polymorphic ventricular tachycardia. In the long QT syndrome, amplification of TDR is often secondary to preferential prolongation of the action potential duration (APD) of M cells, whereas in the Brugada syndrome, it is thought to be because of selective abbreviation of the APD of right ventricular epicardium. Preferential abbreviation of APD of either endocardium or epicardium appears to be responsible for amplification of TDR in the short QT syndrome. In catecholaminergic polymorphic VT, the reversal of the direction of activation of the ventricular wall is responsible for the increase in TDR. In conclusion, the long QT, short QT, Brugada and catecholaminergic VT syndromes are pathologies with very different phenotypes and aetiologies, but which share a common final pathway in causing sudden death.

Cardiac repolarization. The long and short of it

Heterogeneity of transmural ventricular repolarization in the heart has been linked to a variety of arrhythmic manifestations. Electrical heterogeneity in ventricular myocardium is due to ionic distinctions among the three principal cell types: Endocardial, M and Epicardial cells. A reduction in net repolarizing current generally leads to a preferential prolongation of the M cell action potential. An increase in net repolarizing current can lead to a preferential abbreviation of the action potential of right ventricular epicardium or left ventricular endocardium. These changes can result in amplification of transmural heterogeneities of repolarization and thus predispose to the development of potentially lethal reentrant arrhythmias. The long QT, short QT, Brugada and catecholaminergic VT syndromes are all examples of pathologies that have very different phenotypes and aetiologies, but share a common final pathway in causing sudden death via amplification transmural or other spatial dispersion of repolarization within the ventricular myocardium. These same mechanisms are likely to be responsible for life-threatening arrhythmias in a variety of other cardiomyopathies ranging from heart failure and hypertrophy, which may involve mechanisms very similar to those operative in long QT syndrome, to ischaemia and infarction, which may involve mechanisms more closely resembling those responsible for the Brugada syndrome.

Modulation of transmural repolarization

Ventricular myocardium in larger mammals has been shown to be comprised of three distinct cell types: epicardial, M, and endocardial. Epicardial and M cell action potentials differ from endocardial cells with respect to the morphology of phase 1. These cells possess a prominent I(to)-mediated notch responsible for the "spike and dome" morphology of the epicardial and M cell response. M cells are distinguished from the other cell types in that they display a smaller I(Ks), but a larger late I(Na) and I(Na-Ca). These ionic distinctions underlie the longer action potential duration (APD) and steeper APD-rate relationship of the M cell. Difference in the time course of repolarization of phase 1 and phase 3 are responsible for the inscription of the electrocardiographic J wave and T wave, respectively. These repolarization gradients are sensitively modulated by electrotonic communication among the three cells types, [K(1)](o), and the presence of drugs that either reduce or augment net repolarizing current. A reduction in net repolarizing current generally leads to a preferential prolongation of the M cell action potential, responsible for a prolongation of the QT interval and an increase in transmural dispersion of repolarization (TDR), which underlies the development of torsade de pointes arrhythmias. An increase in net repolarizing current can lead to a preferential abbreviation of the action potential of epicardium in the right ventricle (RV), and endocardium in the left ventricle (LV). These actions also lead to a TDR that manifests as the Brugada syndrome in RV and the short QT syndrome in LV.

Effects of Intravenous Magnesium in a Prolonged QT Interval Model of Polymorphic Ventricular Tachycardia Focus on Transmural Ventricular Repolarization

BACKGROUND

This study was performed to clarify the antiarrhythmic effects of magnesium sulfate (Mg(++)) in a prolonged QT interval canine model of polymorphic ventricular tachyarrhythmia (VTA).

METHODS

In six experiments in a canine model of prolonged QT by anthopleurin-A, Mg(++) was administered in boluses of 0.2 mL/kg during repetitive episodes of self-terminating polymorphic VTA or frequent premature ventricular complexes (PVCs). The distribution of ventricular repolarization across the left ventricular(LV) wall and dispersion of transmural repolarization were analyzed before, and 30 and 120 seconds after Mg(++) administration, during ventricular pacing at 100 bpm. Transmural unipolar electrograms were recorded from multipolar needle electrodes, and local activation-recovery intervals (ARI) were measured.

RESULTS

Mg(++) rapidly eliminated self-terminating polymorphic VTA and all isolated PVCs. During ventricular pacing at 100 bpm, Mg(++) caused modest shortening of ARI at all recording sites. Since the magnitude of ARI shortening was greater at mid-myocardial sites than at other ventricular sites, mean transmural ARI dispersion decreased from 80 +/- 22 to 45 +/- 18 ms within 30 seconds after Mg(++) injection. However, this effect was transient, and, at 120 seconds after Mg(++) administration, ARI had increased all sites and transmural ARI dispersion lengthened to 65 +/- 18 ms. Besides suppression of triggered premature activity, homogenization of transmural ventricular repolarization was associated with the antiarrhythmic effects of intravenous Mg(++) in this model.

CONCLUSION

Since these effects were transient, a continuous intravenous infusion of Mg(++) is preferred to prevent recurrences of VTA.

Spatial dispersion of repolarization is a key factor in the arrhythmogenicity of long QT syndrome

INTRODUCTION

The occurrence of significant spatial dispersion of repolarization in vivo as it relates to the mechanism of arrhythmia formation in the long QT syndrome (LQTS) continues to be questioned.

METHODS AND RESULTS

We investigated a guinea pig model of LQT3 using anthopleurin-A (AP-A) to study the contribution of rate-dependent spatial dispersion of repolarization in the intact heart to the arrhythmogenicity of LQTS. Optical action potentials were measured using potentiometric fluorescent dye di-4ANEPPS in Langendorff-perfused hearts with induced AV block. AP-A exacerbated the normal uniform epicardial apex-base action potential duration (APD) gradient, resulting in rate-dependent increased APD dispersion and nonuniform APD gradient. Spontaneous focal premature beats induced functional conduction block along boundaries where large nonuniform APD gradient occurred setting the stage for circulating wavefronts and ventricular tachyarrhythmia (VT). Endocardial ablation abolished spontaneous VT, but nonuniform epicardial APD gradient persisted and could be challenged by a stimulated premature stimulus to induce VT.

CONCLUSION

The study shows that in LQT3, spatial variations in steady-state properties result in zones of nonuniform APD gradients. These provide a substrate for functional conduction block and reentrant excitation when challenged by subendocardial "early afterdepolarization-triggered" premature beats. The study emphasizes the key importance of spatial dispersion of repolarization, whether located in epicardial or intramyocardial layers, in arrhythmia formation in LQTS.

Epicardial activation of left ventricular wall prolongs QT interval and transmural dispersion of repolarization: implications for biventricular pacing

BACKGROUND

Epicardial pacing of the left ventricle (LV) has been shown to prolong the QT interval and predispose to the development of torsade de pointes arrhythmias. The present study examines the cellular basis for QT prolongation and arrhythmogenesis after reversal of the direction of activation of the LV wall.

METHODS AND RESULTS

A transmural ECG and transmembrane action potentials were simultaneously recorded from epicardial, M, and endocardial cells of arterially perfused canine LV wedge preparations. QT interval increased from 297.6+/-3.9 to 314.0+/-5.7 ms (n=12; P<0.001) and transmural dispersion of repolarization (TDR) increased from 35.5+/-5.2 to 70.3+/-6.2 ms (n=12; P<0.001) as pacing was shifted from endocardium to epicardium. Conduction time between M and epicardial cells increased from 12.1+/-1.2 to 24.2+/-1.5 ms (n=12; P<0.001). Amplification of TDR was further accentuated in the presence of rapidly activating delayed rectifier potassium current blockers (E-4031 and cisapride), increasing from 50.5+/-7.6 to 86.1+/-6.2 ms (n=8; P<0.01). Torsade de pointes arrhythmias could be induced during epicardial, but not endocardial, pacing of LV in the presence of rapidly activating delayed rectifier potassium current blockade.

CONCLUSIONS

Reversal of the direction of activation of the LV wall, as occurs during biventricular pacing, leads to a prominent increase in QT and TDR as a result of earlier repolarization of epicardium and delayed activation and repolarization of the midmyocardial M cells. The increase in TDR creates the substrate for the development of torsade de pointes under long-QT conditions.

Ventricular gradient and nondipolar repolarization components increase at higher heart rate

Differences in action potential duration reflect differences in ion channel properties. These properties also determine rate dependence of action potential duration, and transmural dispersion was confirmed experimentally to increase with cycle length. While several electrocardiographic indexes characterizing repolarization abnormalities have been proposed, studies of their heart rate dependence are missing. This study therefore investigated rate relationship of two repolarization descriptors, namely, the so-called total cosine of the QRS-T angle (TCRT), proposed to characterize global repolarization heterogeneity, and the so-called relative T wave residuum (TWR), linked to regional repolarization dispersion. During 24-h holter recordings in 60 healthy subjects (27 males), a 12-lead ECG was obtained every 30 s. RR intervals, QT intervals, and TCRT and TWR were calculated in each ECG and averaged over RR interval bins ranging from 550 to 1,150 ms in 10-ms steps. Women had uniformly greater TCRT and TWR values than men did over the entire range of investigated RR intervals. Whereas the TCRT in both sexes showed marked rate dependence with higher values at long RR intervals (550 vs. 1,150 ms: women, 0.46 +/- 0.31 vs. 0.76 +/- 0.18, P = 9 x 10(-7); men, 0.08 +/- 0.45 vs. 0.49 +/- 0.35, P = 9 x 10(-8)), the rate dependence of TWR was more marked in women than in men, showing higher values at shorter RR intervals (550 ms vs. 1,150 ms: women: 0.29 +/- 0.14% vs. 0.08 +/- 0.06%, P = 2 x 10(-8); men: 0.14 +/- 0.12% vs. 0.04 +/- 0.02%, P = 2 x 10(-15)). This suggests that both global and regional repolarization heterogeneity are increased at faster heart rates. Whereas in women at all heart rates the sequence of repolarization more closely replicates the sequence of depolarization, localized repolarization is more heterogeneous than in men especially at fast heart rates.

Cellular basis and mechanism underlying normal and abnormal myocardial repolarization and arrhythmogenesis

Regional differences in repolarization characteristics of distinct cell types are responsible for the inscription of the J wave and T wave of the electrocardiogram (ECG). Amplification of these electrical heterogeneities contributes to the development of a variety of cardiac arrhythmias. This brief review examines the ionic and cellular basis for these heterogeneities and their role in the Brugada and long-QT syndromes. Both cases involve an accentuation of transmural dispersion of repolarization (TDR). In the case of the Brugada syndrome. TDR is accentuated as a result of a preferential abbreviation of the right ventricular epicardial action potential, whereas in the long-QT syndrome, accentuation of TDR is secondary to a preferential prolongation of the action potential of the M cell.

A method for heart rate-corrected estimation of baroreflex sensitivity

OBJECTIVE

The relationship between the prevailing heart rate (HR) and the baroreflex sensitivity (BRS) is described in the present study together with a method for individual HR-corrected estimations of BRS.

DESIGN

HR and BRS, determined with the sequence method, were measured in ten young healthy subjects during rest, stress, standing and bicycle exercise, i.e. at a wide range of HRs.

RESULTS

BRS decreased exponentially with increasing HR. The relationship between the natural logarithm of BRS and HR was linear in each individual and could be described by the equation of a straight line. The equation describing the individual BRS-HR relationship could be derived either from BRS and HR measured during steady-state conditions or from the slope and average HR of the individual sequences occurring throughout the experimental protocol. The latter method was preferable since it did not require recordings during steady-state conditions. In order to eliminate the influence of differences in HR on BRS when comparing BRS between subjects, the equation describing the individual BRS-HR relationship was used to calculate BRS at a HR of 60 bpm, BRS(60), which ranged from 9.5 to 30.1 ms/mmHg for the 10 subjects.

CONCLUSIONS

Considering the dramatic effect of a small difference in HR on BRS, especially at lower HRs, BRS should be estimated at a wide range of HRs in order to determine the HR-corrected BRS from the individual HR-BRS relationship. Otherwise, comparisons of BRS between different individuals, study groups or following drug treatment or other interventions would be highly dependent on differences in HR and thereby easily misinterpreted.

Electrocardiographic and hemodynamic effects of cisapride alone and combined with erythromycin in anesthetized dogs

The cardiovascular effects of cisapride administered intravenously at escalating doses with and without pretreatment with erythromycin were evaluated in morphine/chloralose anesthetized dogs. Dogs were instrumented to permit simultaneous recording of ECGs, left ventricular (LVP) and aortic (AoP) pressures, as well as programmed electrical stimulation (PES). Escalating intravenous doses of cisapride from 2 to 8 mg/kg (four times the recommended therapeutic dose) increased the heart rate (HR) and prolonged the corrected QT interval (QTc) (p < 0.05) compared to controls. Pretreatment with erythromycin failed to enhance the effect of cisapride on either HR or QTc. Cisapride with or without erythromycin pretreatment had no effect on AoP, but depressed indices of left ventricular contractility (dP/dt(max) decreased while PEP/ET increased) compared to controls. No dogs developed spontaneous arrhythmias, and arrhythmias were not inducible by PES. Cisapride with or without erythromycin pretreatment altered the orientation of the T-wave vector (p < 0.05) compared to controls, indicating a primary effect of cisapride on ventricular repolarization. The QTc and T wave changes observed were consistent with the known action of cisapride on canine I(Kr) channels.

Block of HERG-carried K+ currents by the new repolarization delaying agent H 345/52

INTRODUCTION

The aim of this study was to analyze the block of HERG-carried membrane currents caused by H 345/52, a new antiarrhythmic compound with low proarrhythmic activity, in transfected mouse fibroblasts.

METHODS AND RESULTS

Using the whole-cell configuration of the voltage patch clamp technique, it was demonstrated that H 345/52 concentration-dependently blocked HERG-carried currents with an IC50 of 230 nM. H 345/52 preferentially bound to the open channel with unusually rapid kinetics and was trapped by channel closure. Voltage-independent behavior of H 345/52 was observed during both square-pulse and action potential clamp protocols. In contrast, the Class III agents dofetilide (10 nM) and almokalant (250 nM) demonstrated significant membrane potential-dependent effects during square-pulse clamp protocols. When using action potential clamp protocols, voltage dependence was seen with dofetilide but not with almokalant. Mathematical simulations of human ventricular action potentials predicted that the different voltage-dependent behaviors would not produce marked variations in action potential duration prolongation patterns.

CONCLUSION

We propose that block of IKr is the principal mechanism by which H 345/52 delays repolarization in human myocardium. The voltage independence of HERG/IKr block is unlikely to underlie the low proarrhythmic potential, and ancillary effects on other membrane currents must be considered.

Electrotonic cancellation of transmural electrical gradients in the left ventricle in man

Myocardial cells isolated from different depths of the ventricular wall show substantial differences in action potential duration. Whether these electrophysiological differences are present in vivo when the cells are well coupled is a subject of ongoing controversy. This article provides a brief review and includes experimental evidence derived from patients undergoing cardiac surgery.

Transmural heterogeneity of the action potential configuration in the feline left ventricle

There are M cells in the canine, rabbit, guinea pig, and human left ventricle (LV), but it is not known if they are present in the feline LV. Arterially perfused feline LV preparations were used for the recording of transmembrane action potentials from the epicardium (Epi), midmyocardium (M) and endomyocardium (Endo) under control conditions (n=12) and in the presence of I(Ks) blocker (chromanol 293B: 10 micromol/L, n=6) or I(Kr) blocker (E-4031: 2 micromol/L, n=6). The steady-state action potential duration at 90% repolarization and cycle length (APD90/CL) relation was obtained and fitted by the hyperbolic function APD(90) = CL/[(a x CL) + b]. In control, the shortest and longest action potential duration (APD) were observed in Epi and M, respectively, and the APD(90)/CL-relation curve was steeper in the M or Endo than in the Epi. Chromanol 293B prolonged APD in Epi, but not in M or Endo, resulting in no significant difference of the APD(90)/CL-relation curve among the 3 regions. E-4031 markedly, but homogeneously, prolonged APD in all regions, giving rise to decreased transmural dispersion of repolarization. In conclusion, there exists an M cell layer with a longer APD than the Epi and Endo layers and there is transmural electrical heterogeneity in the feline LV; however, the response to I(Kr) blocker is different from that of the canine LV probably because of species differences in the I(Kr) and I(Ks).

Transmural heterogeneity of calcium handling in canine

Spatial heterogeneity of the action potential and its influence on arrhythmia vulnerability is known. However, heterogeneity of intracellular calcium handling and, in particular, its effect on the electrophysiological substrate is less clear. Using optical mapping techniques, calcium transients and action potentials were recorded simultaneously from ventricular sites across the transmural wall of the arterially perfused canine left ventricular wedge preparation during steady-state baseline pacing and rapid pacing. During baseline pacing, the decay of intracellular calcium to diastolic levels and calcium transient duration were slower (70%, P<0.005) and longer (20%, P<0.005), respectively, closer to the endocardial surface compared with the epicardial surface. Tissue samples isolated from the left ventricular wall demonstrate that sarcoplasmic reticulum Ca2+ ATPase (SERCA2a) expression was significantly less in the subendocardial and midmyocardial layers compared with the subepicardial layer. In contrast, no significant difference in the transmural expression of Na+-Ca2+ exchanger was observed. During rapid pacing, calcium transient alternans and increased levels of diastolic intracellular calcium were significantly greater (P<0.01) closer to the endocardium (101%+/-62% and 41%+/-15%, respectively) compared with the epicardium (12%+/-7% and 12%+/-14%, respectively). In conclusion, cells closer to the endocardium exhibit a slower decay of intracellular calcium compared with cells near the epicardium, which may be due in part to reduced expression of SERCA2a. As a possible consequence, calcium transient alternans and increased diastolic levels of intracellular calcium may occur preferentially closer to the endocardial surface.

Refractory patterns and susceptibility to drug-induced polymorphic ventricular tachycardias in dogs with chronic atrioventricular block: relation to the type of anesthesia

Controversy exists as to the homogeneity of repolarization throughout the canine ventricular wall in vivo. The type of anesthesia has been shown to affect regional differences in monophasic action potential duration and the inducibility of polymorphic ventricular tachycardias (PVTs) in normal canine hearts. This study was conducted to determine refractory patterns and arrhythmia susceptibility in relation to halothane or pentobarbital anesthesia in dogs with chronic atrioventricular block and biventricular hypertrophy. In 12 dogs with chronic atrioventricular block, 60 needle electrodes (12 mm long, four bipolar electrodes, interelectrode distance of 2 mm) were inserted into the left and right ventricle. Six dogs were anesthetized with pentobarbital and six with halothane. Effective refractory periods (ERPs) were determined along 14 randomly selected needles at baseline and after application of almokalant (0.34 mmol/kg) (basic cycle length 1,000 ms, extrastimulus technique). At baseline and on almokalant, ERPs were uniform, independent of the type of anesthesia. With halothane anesthesia, ERPs were significantly longer under both conditions. Almokalant induced not only a prolongation of ERP in both groups but also a significant increase in transmural dispersion of ERP and in maximum dispersion of ERP. However, local refractory gradients were not specific to any muscle layer and did not seem to be related to the occurrence of PVTs. Almokalant did not induce arrhythmias in any dog in the pentobarbital group, but in four of six animals in the halothane group, apparently due to the more marked prolongation in ERP. Independent of the type of anesthesia, hypertrophied hearts of dogs with chronic atrioventricular block exhibit uniform refractory patterns. Longer ERPs with a comparable degree of dispersion on halothane are associated with a high incidence of drug-induced PVTs, whereas shorter ERPs on pentobarbital seem to prevent arrhythmia induction.

Unique topographical distribution of M cells underlies reentrant mechanism of torsade de pointes in the long-QT syndrome

BACKGROUND

Specific ion channel mutations underlie the congenital long-QT syndrome (LQTS). However, the mechanisms by which dysfunction at the molecular level translates into functional electrical instability leading to torsade de pointes (TdP) in LQTS are poorly understood.

METHODS AND RESULTS

The cellular basis of TdP was investigated using a novel approach of transmural optical imaging in the canine wedge preparation (n=14). The spatial organization of repolarization and arrhythmogenesis were determined in a surrogate model of LQT2. Action potentials were recorded simultaneously from 128 sites spanning the transmural wall of the left ventricle. In LQT2, QT interval prolongation was paralleled by an abrupt rise in transmural dispersion of repolarization (DOR) from 2.7 plus/minus 0.9 ms/mm (controls) to 12.2 plus/minus 2.1 ms/mm (LQT2). Islands of midmyocardial (M) cells formed zones of increased refractoriness in LQT2, producing steep spatial gradients of repolarization that were directly responsible for conduction block and self-sustained intramural reentrant circuits underlying TdP.

CONCLUSIONS

These data provide direct evidence supporting the functional expression of M cells in intact myocardium and a central role for M cells in the development of reentrant TdP arrhythmias in LQTS.

Cellular mechanisms underlying the long QT syndrome

QT prolongation is commonly associated with life-threatening torsade de pointes arrhythmias that develop as a consequence of the amplification of electrical heterogeneities intrinsic to the ventricular myocardium. These heterogeneities exist because of differences in the time course of repolarization of the three predominant cell types that make up the ventricular myocardium, giving rise to transmural voltage gradients and a dispersion of repolarization responsible for the inscription of the electrocardiographic T wave. Agents and conditions that reduce net repolarizing current amplify the intrinsic spatial dispersion of repolarization, thus creating the substrate for the development of re-entry. The result is a prolongation of the QT interval, abnormal T waves, and development of polymorphic re-entrant ventricular tachycardia displaying characteristics of torsades de pointes. These conditions also predispose M cells and Purkinje fibers to develop early afterdepolarization-induced extrasystoles, which are thought to trigger episodes of torsades de pointes. Agents that prolong the QT interval but do not increase transmural dispersion of repolarization are not capable of inducing torsades de pointes. The available data suggest that that the principal problem with the long QT syndrome is not long QT intervals but rather the dispersion of repolarization that often accompanies prolongation of the QT interval.

Age, gender, and autonomic tone effects on surface electrocardiographic indices of ventricular repolarization

BACKGROUND

Prolonged QT offset dispersion (QToD), an index of heterogeneity of ventricular repolarization, is thought to be an independent predictor of all-cause and cardiovascular mortality. However the influence of gender and autonomic tone in healthy adults on age-related changes in measures of ventricular repolarization are not well characterized.

METHODS

QToD and T wave complexity were measured in 56 healthy subjects with no detectable heart disease (by echo and stress test)-38 young subjects with a mean age of 28 +/- 4 years and 18 old subjects with a mean age of 71 +/- 7 years. QToD and T wave complexity were computed from 12-lead ECGs using the GE Marquette QT Guard automated analysis program with manual overreading at rest (baseline), following exercise, and double autonomic blockade with atropine and propranolol. Data was analyzed using factorial ANOVA.

RESULTS

Young males had a significantly greater QToD than young and old females at baseline (28 +/- 5 ms, 23 +/- 5 ms, and 22 +/- 5 ms, respectively, P < 0.01), an intrinsic effect independent of changes in autonomic tone. In contrast, females had significantly greater T wave complexity than males following exercise and double autonomic blockade with a definite trend at baseline. There was no correlation between T wave complexity and QToD.

CONCLUSIONS

Age and gender demonstrate a complex interaction on indices of myocardial repolarization with different measures behaving differently. These findings have implications for better understanding age and gender effects on myocardial electrophysiology.